This invention relates generally to the fabrication of machine components and more particularly, to methods of forming a bond coat on a machine component as part of a thermal barrier coating system.
Known turbine blades are coupled to a central hub that is attached to a rotor shaft such that the blades extend generally radially outward from the rotor shaft with respect to a central axis of the hub and shaft. Each blade includes an airfoil. During operation, a high energy driving fluid, such as a combustion gas stream for example, impacts the airfoils to impart a rotational energy to the blades that in turn rotates the shaft.
Because of the high temperatures of known combustion gas streams, some known combustion turbine blades at least include a thermal barrier coating (TBC) system that is formed from a plurality of layers over a substrate surface of the airfoil. The layers may have a variety of material compositions to ensure the TBC systems provide a variety of protective functions. Some known turbine blades have a first layer formed over the airfoil substrate typically using a material often referred to as “bond coat”. Bond coat is a term often used to refer to a variety of materials that form an adherent protective first layer over the substrate and facilitate bonding of a subsequent layer of compatible material to the surface of the layer of bond coat. One example of a TBC system protective function is that TBC systems facilitate shielding airfoils from high temperature combustion gases. More specifically, known TBC systems may reduce substrate temperatures by as much as 100° C. (180° F.), thereby reducing the potential for thermal fatigue and/or creep of the substrate. In addition, the reduced substrate temperature facilitates reducing the potential for thermally-induced oxidation and/or corrosion of the substrate.
During operation, as the airfoils and their TBC systems, are exposed to the hot, and potentially oxidative and/or corrosive environments that typically exist in combustion turbines, the airfoil TBC system may be altered. For example, continued exposure to such environments may adversely impact the thermally grown oxide (TGO) layer and may induce stresses within the laminations of the TGO layer that may cause a premature failure and/or spallation (i.e., sectional removal of a material, or delamination) of the bond coat and/or top coat materials. Spallation of the TBC system may undesirably expose the airfoil substrate to the high temperatures.
Moreover, continued exposure to such environments may also facilitate the diffusion of aluminum from the bond coating. The diffusional loss of aluminum (Al) to the substrate may reduce the concentration of aluminum in the bond coating, thereby reducing the ability of the bond coating to continue generating protective and adherent alumina scale at the TGO layer interface between the bond coat layer and the top coat layer. In addition, the interdiffusion of aluminum may cause a diffusion zone to be formed within the airfoil wall that may adversely affect the substrate properties. For example, the addition of aluminum to the substrate's elemental composition may decrease the substrate fatigue strength of the airfoil wall and/or shorten the life of the airfoil.